Aaron,
For that sized mount, you can get away a thick walled or even 3mm walled 100mm tube and no braces unless you plan on hitting it with a sledge hammer while using it. If you want to put braces / fins, 4mm is plenty. The thickness of the fins won't add much rigidity to the mount but they'll be hard to weld.
Also no problem with bolting it to a slab. I did this many years ago. I used an old surplus truck brake drum as my base flange. I bolted it to the slab. The drum slid with a perfect fit up inside the tube. The tube could rotate on the drum acting as my azimuth adjustment. Three bolts locked the pier to the drum when azimuth was right. The mounting was a 25kg welded steel mount. Much heavier than your 10kg EQ5.
The pier you're describing is suitable for a much bigger scope and mount and overkill for your application. ISSDAOL has his EM400 and Mewlon 300 (~76kg with OTA, mount and counterweights) on a piece of 220mm steel tube bolted to the observatory floor slab. Given the weight of his instrument, he doesn't have fins. You can see it here
http://www.iceinspace.com.au/forum/s...d.php?t=153968
Joe